Process for the reaction of carbon monoxide with steam, with formation of carbon dioxide and hydrogen and use of a catalyst for this purpose.
The present invention relates to a process for the reaction of carbon monoxide with steam, with formation of carbon dioxide and hydrogen at elevated temperature in the presence of iron oxide-containing catalysts.
The process described above is known as a shift reaction and is carried out on a large industrial scale. It is of particular importance in the gasification of coal and in the production of methanol and of ammonia. The state of the art concerning this reaction is described, inter alia, in an abstract in CATAL. REV. SCI. ENG., 21 (2), 275-318 (1980). The reaction is carried out in the presence of catalysts. There are essentially three types of catalysts. One type are those based on iron. These are the so-called high temperature shift catalysts, for which reaction temperatures of about 320.degree. to 450.degree. C. are required. Such catalysts can tolerate small quantities of sulphur in the reaction gas mixture. However, the sulphur content should not exceed about 50 ppm.
Another catalyst type for this reaction are the catalysts based on copper. These are so-called low temperature shift catalysts, which have the disadvantage of being very sensitive to sulphur impurities in the gas mixture. These catalysts are irreversibly poisoned by sulphur compounds. A further disadvantage lies in the fact that they are stable only up to relatively low temperatures. At temperatures above about 260.degree. C., they sinter and lose their activity. Higher temperatures are however desirable for the shift reaction, because the reaction rate rises with increasing temperature. Furthermore in most cases, the low process temperature can only be maintained by the addition of excess steam, which is obviously disadvantageous.
The third type are the catalysts based on cobalt and molybdenum. They are largely insensitive to sulphur compounds. According to the above mentioned literature reference, page 280, the critical sulphur content lies below about 2000 ppm. They are effective at relatively low temperatures, but they have several disadvantages. Owing to their limited availability, they are expensive. The selectivity for the shift reaction is not very good. They catalyse the methanation reaction, which is sometimes undesirable. As a result, it is necessary to employ excess steam in the reactor feed, so as to keep the temperature within the tolerable range. Besides, a certain quantity of hydrogen sulphide has to be present in the feed gas mixture, since, otherwise, the methanation reaction is catalysed to an even larger extent.